Electrochromism refers to a characteristic in which the color of a material is reversibly changed while the electron density is changed with the insertion or desorption of cations in an electrode structure by an electrochemical oxidation/reduction reaction, which is generated according to a change in applied voltage.
A transition metal oxide such as WO3, V2O5, TiO2, and NiO exhibits a hybrid conduction characteristic capable of conducting ions and electrons. When a specific potential is applied to the interface between a thin film electrode of these transition metal oxides in an electrolyte and the electrolyte, an atom such as H+, Na+ or Li+ is charged or discharged. At this time, the color developing-color quenching process is accompanied during the charge-discharge process, so that the transition metal oxides have been actively studied as an electrode material for an electrochemical coloring element. Since a display element using the electrical coloring phenomenon may obtain a desired light transmittance level by changing an externally applied potential, the display element is expected to be used for a special glass such as a curtainless window and an electrochemical coloring display element in the form of a mirror, which uses the same.
A structure of an electrochromic element is schematically illustrated in the following FIG. 1. More specifically, the electrochromic element has a structure in which a first electrode 11, a WO3 thin film 12, an electrolyte layer 3, a LiNiOx thin film 22, a second electrode 21, and a second substrate 20 are sequentially stacked on a first substrate 10.
The WO3 thin film 12 is typically formed by a sputtering process, chemical vapor deposition (CVD), or a sol-gel method. The electrolyte layer 3 may use a solid or liquid state.
When a voltage is applied between the first electrode 11 and the second electrode 21, ions in the electrolyte layer 3 move and thus are reacted in the WO3 thin film as in the following chemical formula, leading to chromism.WO3(transparent color)+xe−+xM+↔MxWO3  (deep blue)
At this time, cations in the electrolyte layer 3 move through cracks or a free volume in the WO3 thin film 12, and a thickness of the WO3 thin film 12 needs to be increased in order to enhance a coloring degree, but there is a problem in that as a moving distance of cations is increased, a discoloration rate becomes slow.
Further, in constituting an electrochromic element, WO3 thin films were prepared by using a sputtering method in the related art. However, the sputtering method has a problem in that the method is not suitable for mass production because process equipment is expensive and a deposition rate is too slow to manufacture a WO3 thin film having a thickness of several hundred nanometers.